It seems that we're *finally* nearing the end of the long blkcg APIcleanup. With all the previous updates, blkcg code is now clean &modular enough to implement per-queue policy activation.

Upto now, blkcg assumed that all policies are active on all queueswhich simply isn't true - cfq policy is only applicable to queueswhich are using cfq as the elevator. Also, for transition purposes,the current implementation clears all !root blkgs blkgs acrosselevator switch and policy [de]registration in racy manner.

This patchset implements per-queue policy activation. Static policyID is replaced with dynamic registration and each policy should beactivated and deactivated explicitly on each queue. On activation,the matching policy data are created on all existing blkgs. Ondeactivation, the matching policy data are removed from all existingblkgs. blkg printing skips blkg-policy combination which is disabledand trying to configure a policy on a blkg which doesn't have thepolicy enabled would return -EINVAL instead of creating an unuseddummy configuration.

This patchset is on top of the current block/for-3.5/core 5bc4afb1ec"blkcg: drop BLKCG_STAT_{PRIV|POL|OFF} macros" and also available inthe following git branch. git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git blkcg-pols